1. Field of the Invention
The present invention relates generally to methods of forming composite structures for scientific and technical uses. More particularly, the present invention relates to methods of bonding materials such as glass, and composites of glass and glass ceramics, or single crystals without the use of bonding agents, at temperatures lower than the melting temperatures of the materials to be bonded. The glass laminates of the present invention are particularly well-suited for use in filter glass and laser glass applications.
2. Description of the Prior Art
In many endeavors of science and industry, instruments and devices utilizing structures comprising glass-containing substances are necessary to produce certain important scientific phenomena and to provide proper and efficient operation of certain scientific and engineering devices. For example, plate glass structures are useful in solid state lasers. Also, adhesively-bonded glass laminates have been employed in filter glasses used in optic systems.
Methods described in the prior art for bonding glass surfaces include holding the surfaces together using adhesive or bonding agents, followed by heating to a molten state. For instance, U.S. Pat. No. 4,149,902 discloses the use of a transparent bonding adhesive. U.S. Pat. No. 3,880,632 discloses the use of a binding layer of silica on one surface, followed by heating by infrared radiation.
Other variants on the general concept of bonding silicon bodies by means of an intermediate layer are those such as disclosed in European Patent Application No. 0 232 935 which suggests bonding of a glass body to a semiconductor body with use of an intermediate thermally grown oxide layer. Similarly, U.S. Pat. No. 4,810,318 discloses forming an optical laminate filter by bonding two glass parts by means of wringing in contact the glass parts through an intermediate vapor-deposited indium-tin oxide layer. The U.S. Pat. No. 4,810,318 also refers to other conventional bonding layers such as an epitaxial monocrystalline layer, for example, Si on GaP or (PbLa)(ZrTi)O.sub.3 on sapphire. Also, U.S. Pat. No. 4,638,552 suggests bonding silicon bodies with use of an intermediate thermal oxide or impurity layer. Lastly, U.S. Pat. No. 4,531,809 discloses an optical waveguide coupling device including a silicon part attached to a polymer film over an intermediate thin metal film.
However, the aforementioned methods are not totally satisfactory since the use of bonding agents may affect optical quality, and present considerable difficulties and inconvenience. For instance, in using a bonding agent, the optical characteristics of the bonding agent would have to be compatible with the materials to achieve optical homogeneity. Further, if the interface between components is contaminated during contact, the bonding agent would prevent easy separation of the surfaces for cleaning and recontacting. If high heating temperatures are used, the high temperatures and molten state make it difficult to (1) control the flatness of the element interfaces, (2) keep the interfaces free of contamination by dust and gas bubbles, and (3) achieve optical homogeneity of the layers because of possible distortions.
Accordingly, given the above-mentioned significant drawbacks associated with the use of intermediate bonding layers in bonding glass parts, other prior art approaches have been attempted. For instance, another method proposed for bonding a laminate assembly involves attaching one substrate of borosilicate glass to a silicon wafer without the use of intervening adhesive layers and heating to a glass transition point or a higher temperature such as seen in European Patent Application No. 0 136 050. The borosilicate glass/single crystal silicon wafer composite was found inferior to a single crystal silicon wafer/single crystal silicon wafer composite for semiconductor pressure transducer applications.
However, no explanation is offered in European Patent Application 0 136 050 for the difference in strength observed between the borosilicate glass/silicon wafer composite and the silicon wafer/silicon wafer composite. It is possible that the borosilicate glass will hydrate more extensively than the silicon surface since this glass contains additional other components such as alkali and borate which also undergo hydration. As a consequence, the hydrated layer of the borosilicate glass can be expected to be thicker and to contain a higher water content than the silicon wafer. The thicker layer at the interface of the borosilicate glass tends to adversely effect the bond strength because hydrogen bonding therein is recognized as giving a weaker bond in comparison to the glass network bond, such as Si--O--Si bonds, at the interface of the silicon wafer/silicon wafer composite.
In the latter situation, water of condensation will remain at the interface because the heating temperature used in European Patent Application 0 136 050 is maintained too low for diffusion to occur.
That is, the single crystal wafer bonding embodiment in European Patent Application 0 136 050 strongly suggests that the presence of oxide films (silicon oxides, dioxides and/or hydroxide groups) on the surfaces to be joined are responsible for increasing the bonding strength of the silicon-silicon composite. These oxide layers may only be one to a few molecular layers thick; however, European Patent Application 0 136 050 indicates that the mechanisms of silanol condensation in the oxide films are operative to effect the bonding of the silicon wafers. Moreover, European Patent Application 0 136 050 states that the silicon bodies were not bonded by mutual diffusion mechanisms.
In a different embodiment, European Patent Publication 0 136 050 also suggests that two glass parts can be joined by merely mutually contacting the glass surfaces at ambient non-heated conditions. In this regard, the reference speculates that alkali metal ions from one glass part are possibly dissolving into the water-adsorbing surface regions of the other glass part. However, this explanation is not considered theoretically plausible given the nature of the alkali metal ions and common glass substrates involved together with the ambient bonding temperature condition. Other prior methods for forming composite glass parts without use of intervening bonding layers are described below.
For example, thin, laminated glass composites have been produced by co-extrusion from slit-shaped orifices followed by the joining of melt-softened glass sheets. This method, however, will not provide the optical homogeneity necessary for laser applications. Further, this method is incapable of producing glass sheets thicker than approximately 5 to 10 mm; and such composites also lack structural rigidity.
The molten glass method involves casting layers of molten glass, one on top of another, and allowing each layer to cool and solidify. However, this method has disadvantages because the flatness of the glass interface and the desire to maintain a contamination-free environment of the glass interface are difficult to control in the molten state. Moreover, since the molten state subjects the layers of the glass composite to thermal stress, optical homogeneity is difficult to maintain.
Techniques for optically contacting glass parts have been considered heretofore. For instance, a method involving optical contacting using bonding agents is disclosed in U.S. Pat. No. 3,565,508. Although this process is simple conceptually, it is more complicated to actually implement due to, among other factors, the pervasive requirement of extreme cleanliness required of glass surfaces to be optically contacted.
Also, most filter glasses need an optical thin film coating. This coating has to be deposited prior to lamination with cement or adhesive because the cement or adhesive would decompose at coating processing conditions. Therefore, great care has to be taken not to damage this coating during the lamination process.
However, as explained below, optical contacting between optically flat surfaces, without bonding agents, is possible. This bonding can be attributed to Van der Waals attractive forces acting at opposing contact points and surfaces. Such bonding remains stable so long as the components of the composite are not subjected to temperature gradients, which cause non-uniform expansion, and resultant stress sufficient to overcome this bond strength. However, the bond may also be broken by inserting a thin strong object, for instance, a razor blade in between the optically contacted surfaces. For reference, see G. W. McLellan et al., Glass Engineering Handbook, 3d ed., which describes heating and cooling of glasses.
As made apparent by the discussions above, there remains a need for high quality large glass or glass-containing structures useful for various technological applications. Conventional optical filter glasses or large glass structures constructed in the desired sizes lack optical quality and structural endurance. Moreover, traditional methods for forming laser quality composites are limited, thus far, to materials capable of remaining stable at high bonding temperatures and pressures. Finally, the use of extraneous bonding agents presents difficulties in contamination control, optical quality and other undesirable effects.
Accordingly, this field of technology has awaited proposals which might offer solutions to the above problems encountered in past attempts to provide composite glass-containing structures. However, the need for providing structures of materials of which the optical properties are being utilized is not limited to glass-containing structures, but also extends to crystalline structures necessary for laser harmonic generation, electro-optical Q-switching, and in moderate to high average power solid state lasers.
Conventional approaches to providing such large crystalline structures for these applications have proven unsatisfactory. For example, traditionally, large single crystal materials have been grown, but this is necessarily a slow and complex process, with the costs increasing as the crystal size increases. Moreover, many crystals can only be grown to a limited size which is frequently smaller than desired for intended applications. For example, Nd:YAG (i.e., yttrium aluminum garnet) crystals, commonly used in lasers, can only be readily grown into only about one inch diameter crystal boules; larger boules suffer from imperfect cores, which effectively reduce the usable dimensions of the boule, and non-uniform dopant concentrations.
Another method of producing large crystalline structures is to construct composites of single crystals, or composites of different crystalline substances. Fabricating composite large single crystals is more cost-effective than growing large crystals, and multiple composites can provide needed structural support to the crystalline structure. However, traditional methods of bonding crystalline materials require relatively high temperatures and/or pressures, so only those crystals which are stable at those bonding temperatures and/or pressures could be produced. For example, the process of epitaxial growth (i.e., the layering of single crystals on tope of a single crystal substrate) requires exposing the substrate to a high temperature flux melt.
More specifically, one method of growing epitaxial layers consists of immersing the substrate crystal into a flux at temperatures of up to about 1400.degree. C. to deposit an epitaxial layer onto the substrate crystal on all of its exposed surfaces. However, it is difficult to mask crystal surfaces on which such an epitaxially grown layer is not desired, since a mask material capable of withstanding the flux temperatures generally does not exist. Moreover, this particular method presents the risk of breakage due to thermal shock and/or stress, and damage or partial loss of substrate material due to the difficulties of controlling the outcome of the epitaxial growth process. This problem is particularly severe when expensive, large single crystal slabs--which take several months to grow--such as, for example, Nd, Cr:GSGG (i.e., gadolinium scandium gallium garnet) are involved.
Accordingly, a need also exists in the art for the provision of a technique to provide large crystalline structures which overcomes and avoids the above-discussed shortcomings of the conventional approaches.